1. Field of the Invention
This invention relates to a novel olefin polymerization catalyst and a process for preparing polyolefins using said catalyst. More specifically, it relates to an olefin polymerization catalyst which includes: (A) a solid catalyst component being prepared by copulverizing a magnesium compound, an aluminum compound, an electron donor and a titanium compound, and (B) an organoaluminum compound. It is also directed to a process for preparing polyolefins, e.g. polyethylene and its copolymers with low solvent extractable, utilizing the aforesaid catalyst.
2. Information Disclosure Statement
Olefin polymerization catalysts are described in the prior art which are obtained by combining a constituent comprising a magnesium halide and a titanium halide with an activating organoaluminum compound. (See: Boor, J. Jr.; “Ziegler-Natta Catalysts and Polymerization”, Academic Press, New York, 1979; Barbe, Pier Camillo; et al. “The Catalytic System Ti-Complex/MgCl2”, Adv. Polym. Sci., 81, 1-81 (1987); Dusseault, John J. A.; Hsu, Cheng C.; “MgCl2-supported Ziegler-Natta Catalysts for Olefin Polymerization: Basic Structure, Mechanism, and Kinetic Behavior”, J. Macromol. Sci. Macromol. Chem. Phys., C33(2), 103-145 (1993)) Nevertheless, the quest for higher catalytic activity coupled with better polymer properties continues. The present invention achieves these two goals.
In the production of polyolefins using Ziegler-Natta catalyst systems, the residual catalyst components go with and remain in polymer product. A high concentration of catalyst residual would not only result in various defects and disadvantages in final product such as coloration and deterioration of molded articles, formation of fish eyes, and yarn breakages and coloration of fibrous articles, but also causes such undesired phenomena in polymer processing machines as rusting and corrosion. However, it is extremely difficult and costly to remove such catalyst residual from the resulting polymer product, and it is practically impossible to accomplish such removal. The only possible way to overcome such problem in polyolefin industry has been through new catalyst development to improve catalyst efficiency. The catalyst efficiency is reflected by catalyst activity, which is normally measured by the grams of polyolefin produced per gram of catalyst containing titanium or other transition metal. The higher the activity, the lower the amount of residual catalyst components remaining in the resulting polymer product. If the activity is high enough, the catalyst residue can be reduced to such level that its existence will have little or no adversary effect on polymer properties and processing machines. While almost all existing commercially available olefin polymerization catalyst systems are efficient enough to eliminate the step of removing catalyst residue from the resulting polymer product, a considerably large amount of additives is still required to neutralize the residual catalyst components in order to achieve good quality polymer products. In addition, the additives for such purpose are rather expensive. Therefore, it is very desirable to further improve catalyst efficiency and consequently to completely eliminate the problems caused by catalyst residue without too many additives.
During olefin polymerization, there are chances of formation of low molecular weight polyolefin polymers or oligomers, which are normally dissolved in hydrocarbon solvents under high temperatures and characterized as solvent extractable. In this patent, we define solvent extractable as the low molecular weight polyolefin polymers or oligomers which are dissolved in an extraction hydrocarbon solvent such as hexane and heptane under continuous refluxing the extraction solvent.
The formation of solvent extractable during olefin polymerization processes will impair reactor heat transfer and dryer efficiency and consequently lower production efficiency. In addition, the solvent extractable in polyolefin resins generally impair transparency, impact resistance and blocking property of processed resin. Furthermore, the solvent extractable often causes a lot of troubles during polymer processing, such as smoking and die buildup. However, to remove such wax and oligomers from the resulting polymer products requires additional expensive equipment such as filtration and adds more complexity in the polymer production process. Therefore, it is very desirable to have a catalyst system capable of producing polyolefins with low solvent extractable formation during polymer production process.
It is also desirable to produce polymer powder with as high bulk density as possible. Low polymer bulk density will cause a lot of reactor operational problems, such as poor heat transfer, poor dryer and centrifuge efficiency, lower production rate, and clogging of transportation equipment, etc. Thus, a high bulk density and a good flow property of polymer powder are desirable from the viewpoint of stability and efficiency of operation.
It is also very desirable that a catalyst system for olefin polymerization have a good comonomer incorporation ability to reduce the usage of the often more expensive comonomer, and in the meantime, to tailor polymer molecular structure and composition, and consequently to achieve better polymer properties.
U.S. Pat. No. 3,991,260 discloses a catalyst being prepared by copulverizing a magnesium dihalide, an aluminum alkoxide and a titanium or vanadium compound with a considerably higher polymerization activity and polymer bulk density. However, this catalyst has been found to produce polyolefin with a high content of solvent extractable in olefin polymerization, and the catalyst activity and polymer bulk density are still desired to be improved.
It is known in the art that improved olefin polymerization catalysts can be prepared by co-milling magnesium dihalide, an organic ester and titanium tetrachloride. The organic ester such as ethyl benzoate is found to decrease the crystallite size of magnesium dihalide and introduce crystal distortions. When used to polymerize propylene, these catalysts provide higher isotactic index, or lower amount of atactic soluble polypropylene.
U.S. Pat. No. 4,069,169 discloses a catalyst prepared by milling magnesium chloride, ethyl benzoate, and titanium tetrachloride followed by the treatment with titanium tetrachloride, optionally in the presence of a hydrocarbon solvent.
U.S. Pat. No. 4,143,223 discloses a solid halogen-containing titanium catalyst component obtained by reacting a mechanically copulverized solid product in the absence of mechanical pulverization with a titanium compound which is liquid under the reaction conditions, said mechanically copulverized product being derived from a magnesium compound, an organic acid ester, and an active hydrogen-containing compound selected from the group of alcohols and phenols.
U.S. Pat. No. 4,450,242 discloses a catalytic component containing titanium obtained by: (i) copulverizing a substantially anhydrous magnesium compound containing halogen or manganese compound containing halogen with a phenol, an organic polymer containing silicon, a titanium halide, and an electron donor compound to produce a copulverized product, and (ii) reacting the copulverized product with a liquid titanium compound containing halogen.
U.S. Pat. No. 4,347,158 discloses that an enhanced support made of the materials comprising a support base, an inorganic Lewis acid and an electron donor is combined with an active transition metal compound and optionally a second electron donor to form the catalyst component. In the preferred embodiment of U.S. Pat. No. 4,347,158 a catalyst is prepared by copulverizing magnesium dichloride, aluminum trichloride, anisole, with a complex of titanium tetrachloride and ethyl benzoate. This patent specifically requires that the solid catalyst component is an inorganic Lewis acid and preferably as inorganic aluminum trichloride, whereas, in the present invention, an organic aluminum compound is used as a copulverizing component and preferably as aluminum trialkoxide.
Further, there are no indications that these catalysts described in the U.S. Pat. No. 4,069,169, No. 4,143,223, No. 4,450,242 and No. 4,347,158 can produce polyethylene and its copolymers with low solvent extractable. Furthermore, the polymerization activities of these catalysts and polymer bulk densities are still unsatisfactory and desired to be improved. In addition, those catalyst systems exhibit a decay olefin polymerization kinetic characteristics, which has a high initial catalyst activity and the activity decreases rapidly with time. Such a decay kinetic behavior is not desired for polyolefin production process. The present invention overcomes all of these shortcomings of the prior art.
U.S. Pat. No. 4,022,958 relates to a process for preparing polyolefins using an improved polymerization catalyst. According to the invention, there is provided a process for preparing polyolefins by polymerizing or copolymerizing olefins using a catalyst prepared from a component with a titanium compound and/or a vanadium compound supported on a solid carrier and an organoaluminum compound and/or an organozinc compound, said solid carrier comprising a member selected from the group consisting of a reaction product A obtained by reacting (1) an organic carboxylate salt of magnesium and (2) an aluminum compound represented by the general formula Al(OR)3 wherein R is an organic radical containing from 1 to 20 carbon atoms under such a condition that the aluminum compound is present in excess of a (2):(1) molar ratio of at least 1:6:1 and a reaction product B obtained by treating said reaction product A with a halogenating agent.
U.S. Pat. No. 4,180,636 demonstrates a process for polymerizing or co-polymerizing propylene in the presence of a catalyst consisting essentially of (A) a solid catalyst component which is prepared by contacting a copulverized material obtained by copulverizing a magnesium dihalide compound together with an acyl halide with (2) a mixture or addition-reaction product of a tetravalent titanium compound containing at least one halogen atom with at least one electron donor compound selected from the group consisting of organic compounds containing a P—O bond, organic compounds containing an Si—O bond, ether compounds, nitrite ester compounds, sulfite ester compounds, alcohol compounds, phenol compounds and naphthol compounds,
(B) a trialkyl aluminum compound, and (C) a carboxylic acid ester compound.
U.S. Pat. No. 4,439,537 describes a process for the preparation of the former catalyst component of a catalyst for the polymerization of olefins composed of a titanium-based catalyst component and an organo aluminum compound is disclosed. The former catalyst component can readily be produced by contacting (a) a fatty acid salt of magnesium, (b) an electron donor compound and (c) a titanium halide. In the polymerization of olefins by use of the former catalyst component, both the amount of catalyst residues in the produced polymer and halogen content therein are greatly reduced with high polymerization activity per unit weight of the former catalyst component and with high yield of stereoregular polymer.
U.S. Pat. No. 4,552,859 describes how isotactic index improvement is achieved for C3 and higher alpha olefins in systems containing a catalyst component comprising titanium supported on a magnesium halide support. The titanium component is formed by copulverizing the magnesium halide with one or more electron donors followed by treatment with liquid titanium halide. The improvement is achieved by using a dialkylaluminoxane component with the trialkylaluminum co-catalyst normally used. In slurry polymerizations, the isotactic index of the polymer has been improved.
U.S. Pat. No. 4,673,661 describes catalysts for polymerization and copolymerization of olefins to form polymers having a high degree of isotacticity and fast rate of crystal transformation. The catalysts embody a component made by chlorinating a magnesium alkyl with chlorine or a mixture of chlorine and alkyl chloride to form a carrier, contacting the carrier first with liquid TiCl4 then with a Lewis base, and after that at least once with TiCl4 in the absence of Lewis base.
Notwithstanding the prior art, the present invention is neither taught nor rendered obvious thereby.